Fascinating finding from the folks at the NPD Group: Americans now stream as much music as they download, at least on computers.

NPD (via Evolver.fm) says downloaded music accounted for 30 percent of listening on Macs and PCs in August, compared to 29 percent in March. Over the same period, streams increased from 25 percent to 29 percent of computer listening behavior. Judging from those statistics, streams could leap ahead of downloads next time NPD takes measurements.

It’s still too early to start the MP3 death watch. Streaming on portable devices won’t become the norm until the majority of people own smartphones.

And smartphones pose their own challenges. Right now, streaming music on a computer is natural. If you’re on the web a lot, you probably come across YouTube music videos, MySpace pages and music search results in Google and Bing frequently. Those options aren’t always available on phones, because some record labels block music videos from mobile YouTube, MySpace music requires Flash and song streams don’t come up in mobile searches. Music streaming on phones is mainly limited to subscription music services, and then you need a reliable 3G connection to make it work.

But the latest data from NPD is evidence, at least, that people really like to stream music. There’s more: In July, NPD estimated that 7 million to 8 million U.S. iTunes users have a strong interest in subscription music streaming, and found that a quarter of survey respondents would like a free, cloud-based music option.

Apple hasn’t jumped into streaming music yet, and Google has only dabbled. There are rumors that both companies might introduce cloud music services, and it’s only a matter of time as streaming music grows in popularity. That’s when you can start worrying about the downloaded MP3.